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nderstanding the mechanical properties of technologically advanced materials from quantum mechanical predictions based oil electronic structure calculations remains one of the most challenging problems in modern computational materials science. In this paper, we illustrate this challenge from our current investigations oil dislocation behaviour ill bee transition metals that are promising candidates for materials subject to fast neutron irradiations in future fusion power plants. Starting with the relationship between the brittleness and the negative Cauchy pressure of elastic constants ill materials within the so-called Harris-Foulkes approximation to the density functional theory (DFT), we briefly discuss the importance of the generic form of interatomic potentials in order to reproduce a correct Cauchy pressure. The latter in turn plays an important role in predicting dislocation properties in fee iridium and therefore a I lows us to explain experimental observation (if the intrinsic brittleness of this material. We then investigate the behaviour of the (1/2)[111] screw dislocation that controls plastic deformation in bee metals from atomistic simulation. Here we show the atomic phenomena associated with the non-planar core structure of dislocations in bee iron from the Stoner tight-binding bond model. The crucial point comes from the accurate evaluation of forces implemented within the charge neutrality conditions in the treatment of the spin-polarized dependence in the electronic structure calculations. In agreement with DFT studies, the magnetic bond-order potentials predict a non-degenerate core structure for screw dislocations in Fe. Finally, a new analytic expression has been derived for the migration energy barrier for the one-dimensional (11 D) motion of crowdions, which are the most,;table self-interstitial atom (SIA) defects predicted by our DFT calculations. Importantly, the latter study is strongly supported by the recent observation of ID diffusion of nanometer-sized dislocation loops, observed very recently under in situ electron microscope irradiation for bee transition metals.